A blast that ripped through a residential street in north-west London, killing a man and injuring a 17-year-old girl, may have been caused deliberately by a group of teenage girls settling a feud with one of the occupants, police said yesterday.

Two girls were seen pouring a purple liquid through the letterbox of a ground floor flat in Stanley Road, Harrow, hours before the explosion levelled three terrace houses, killing Emad Qureshi, 26, and burying a teenage girl in rubble on Wednesday evening.

The girl, who police believe was the target of the attack, was hauled from the debris and taken to hospital, where she is being treated for severe burns.

At the scene yesterday, Detective Chief Inspector Colin Sutton cast doubt on an early theory that the blast was caused by a gas leak, saying the focus was on tracing the purple liquid.

Sutton said: "We still haven't ruled out a gas explosion but experts say it is unlikely to be the cause. We are aware that at some point during the course of Wednesday there was a substance put through the letterbox of No 21. There may well be a connection between this liquid and the cause of the explosion. If it were a volatile liquid and in a confined space it could evaporate into an explosive mixture."

At about 11am on Wednesday, police received a complaint about two girls causing a disturbance in Stanley Road. Investigating officers believe the 17-year-old victim knew the two girls who had been causing the disturbance, perhaps from school or college.

"They are obviously people she knows, who were there to do her some harm. Whoever did this, if it is what we think it is, it was targeted - it was not a random person going around blowing up flats in Harrow," Sutton said.

Forensic experts were last night hunting for clues to the cause of the blast but were unable to access the epicentre, believed to be 21 Stanley Road.

Explosives expert Professor Hans Michels, from Imperial College London, said the liquid could be an agricultural diesel which causes an explosive vapour cloud if left in a confined space for several hours.

He added: "If this was diesel that was left for 10 hours then it would fill a large area, which definitely could cause a huge explosion. Something as simple as central heating going on or a tiny spark could be enough to set it off."